Does anyone know?

The Source is just an aspect of the Presence. Metaphorically, The Presence is a house, while The Source is a brick.

No. Nobody does. They fulfill more or less the same purpose in comics, that of an omnipotent entity which created this universe and everything in it. It could be that the Source is an aspect of the Presence, or it could be the other way around, or it could be that they are both aspects of the Creator.

@DarkKnight96000:

The Source exists in the center of every universe(and allows people to develop superpowers which defy physics)

while there is only one presence.

I always looked at The Presence as the supreme being and the Source as the energies that stem from Him

The Presence seems to be a superpersonal being while The Source is a subpersonal substance.

It's all up to what you interpret them as personally i feel the source is another aspect of the presence much like the voice, the hand, the great evil beast and all that jazz.

The Presence is God of the DCU/Vertigo. The Source is the source of all existence.

Same thing